Batteries, such as lithium ion batteries, have a small size, high energy density, low pollution, and other advantages. For the packaging of many lithium ion batteries, the metal shell is usually covered with a polyethylene terephthalate (PET) or a plastic film. The film provides an attractive appearance and contains labels or printed identification marks. The film also protects the main body of the batteries from abrasion. However, this kind of packaging can not protect the batteries from overheating. In addition, it is a known method to install a pressure relief valve on the shell of the battery cell to improve safety. When the internal pressure is high, the valve will open and release the internal pressure to prevent explosion. However, the hot material in the battery will spray out via the valve and contaminate the outer environment. When the internal flammable materials spray out and contact with air, it may cause fire.